The present invention relates generally to a method for forming an effective scale on piping and other facilities which handle corrosive water streams. In one aspect, the invention relates to a method for protecting mild steel facilities from corrosion by water containing dissolved salts (e.g. sea water). In a particular embodiment of the invention, the method relates to a method of protecting mild steel facilities from the corrosive effects of water containing oxidizing treating agents. In still another aspect of the invention, the method relates to a method of protecting mild steel from the corrosive effects of aqueous solutions of chlorine dioxide.
Aqueous solutions of chlorine based oxidizing agents have long been used in the treatment of industrial water, municipal water, and oil field water flooding operations for purification of the water, odor control, and as a biocide. Chlorine dioxide, for example, has been used in the treatment of water containing phenol, the treatment of water as a biocide for oil field water flooding system, and the purification of industrial and municipal water systems.
Although chlorine dioxide and the other oxidizing agents are quite effective in the treatments noted above, they present a serious problem because of their high corrosivity. The use of organic corrosion inhibitors has not proven cost effective. Moreover, the use of corrosion-resistant alloy steels in many operations is cost prohibitive.
As will be described in detail below, the present invention involves a method for forming a protective scale coating on the interior surface of the pipe or other facilities. The protective coating is an inorganic scale inhibitor formed in situ. The scale results in part from the corrosion reaction products formed during the process of induced corrosion described below. The present method does not involve the use of any of the organic film forming corrosion inhibitors, but does not preclude their use.
The use of controlled calcium carbonate scale for corrosion protection has long been known. An early article published in Industrial and Engineering Chemistry in 1945 and entitled "Corrosion Prevention by Controlled Calcium Carbonate Scale" states that "the deliberate production of calcium carbonate scale to protect water distribution systems is carried out by many municipal water treatment plants and has been applied widely in the industry." The control of the scaling rate and thickness is by temperature, pH, and addition of lime and acid. The scale described in this process, however, is crystalline scale formation.
A 1936 article entitled "The Analytical Control of Anti-corrosion Water Treatment", authored by W. F. Langelier recognized that certain products of the corrosive action can be deposited on the boundary surface as a scale thereby protecting the surface from corrosive effects of the water. The article further describes a method of adding lime to increase the concentration of calcium ions and carbonate ions simultaneously. The deposition of the calcium carbonate as a protective scale on the pipes appear to be crystalline deposition resulting from supersaturated solutions.
U.S. Pat. No. 4,678,685 discloses a method wherein water supersaturated with calcium carbonate precipitates crystalline calcium carbonate on the metal surfaces. The suspended calcium carbonate presents operational problems since excessive amounts of the suspended calcium carbonate must be removed during the filming step.
U.S. Pat. No. 5,051,281 discloses a method of developing calcium carbonate scale on metal using powdered lime. The scaling of the pipe relies on the use of particles of solid lime in the water to form a precipitate of calcium carbonate, which adheres to the pipe wall.
U.S. Pat. Nos. 5,084,210 and 5,064,612 disclose organic corrosion inhibitors used in connection with the treatment of aqueous solutions of chlorine dioxide to protect metal surfaces against corrosion.
U.S. Pat. No. 4,945,992 discloses the use of sodium dichromate as a corrosion inhibitor for chlorine dioxide systems. Due to environmental concerns, it is not possible to use chromates in many systems.